1. Field of the Invention
The invention generally relates to whiter photoinitiated dental resins and composites, and to methods for making same. More specifically, the invention relates to light-cure dental compositions including at least one photopolymerizable monomer and a photoinitiation system including (a) a 1-aryl-2-alkyl-1,2-ethanedione and (b) a rigid 1,2-dione in a weight ratio of (a):(b) in a range of about 1:20 to about 20:1, wherein the mixture is present in an amount sufficient to achieve a degree of double-bond conversion (DC) of at least 50%.
2. Brief Description of Related Technology
There is a consensus that improved conversion of double bonds during photopolymerization is critical for the optimization of mechanical properties (Ferracane and Greener, 1986; Ferracane et al., 1997; Peutzfeldt and Asmussen, 1996, 1992), biocompatibility (Rietschel, 1986), and color stability (Imazato et al., 1995) of light-activated dental resins. Photopolymerization implies both the light-induced increase of molecular weight by monomer to polymer conversion, as well as crosslinking of developing or preexisting macromolecules.
Photopolymerization reactions commonly are initiated by free-radicals formed by photosensitizers (also referred to hereinafter as photoinitiators). These photosensitizers typically posses a carbonyl group having a non-bonding electron capable of being promoted into the .pi.* anti-bonding orbital by absorption of light of the quantum mechanically allowed wavelength. This electron promotion leads to production of a pair of free radicals, either by intramolecular cleavage (e.g., as with benzoin ethers such as benzoin methyl ether) or by proton abstraction from a labile site (e.g., an .alpha.-alkylamine group on amines such as N,N-dimethylamino ethylmethacrylate) with photosensitizers such as camphorquinone (CQ). Generally, photoinitiators should absorb and photoinitiate polymerization reactions in the visible light spectrum, such as at a wavelength of about 470 namometers (nm) for CQ.
Proton abstraction can be made highly efficient by formation of a complex between the photoexcited sensitizer and an electron-donating (reducing) agent, such as a tertiary amine. The complex is referred to as an "exciplex." Proton abstraction occurs within this exciplex which then breaks down to form free radicals (Oster and Yang, 1968; Hutchison and Ledwith, 1974; Ledwith, 1977). Aliphatic compounds containing two or more vicinal carbonyl groups have also been used as photosensitizers. Diacetyl, for example, absorbs in both the near-ultraviolet and blue regions of the spectrum (up to 467 nanometers (nm)) and has been used as a sensitizer for polymerization of methyl methacrylate with visible light (Gladyshev and Rafikov, 1962).
Even though a variety of compounds can act as initiators in the visible light region (Pummerer and Kehlen, 1933; Oster 1954, 1958; Oster et al., 1959; Fouassier, 1993), most investigations concerning dental resins have utilized CQ as a photosensitizer (Linden, 1993; Taira et al., 1988) in combination with a variety of reducing agents (Kubo, 1989; Nikaido, 1989; Kadoma and Imai, 1990; Yoshida and Greener, 1993). CQ is an alpha dicarbonyl that absorbs light having a wavelength of about 468 nm and, therefore, forms a very effective photoinitiator system when combined with an electron donor. However, CQ is inherently yellow, which causes problems in color matching. This, in turn, places practical limits on the concentration of CQ in a dental resin and, consequently, limits the degree of polymerization and depth of cure that can be attained. Efforts to improve the curing system have investigated the use of alternative photosensitizers (Peutzfeldt and Asmussen, 1992, 1992; Peutzfeldt, 1994, 1994; Peutzfeldt and Asmussen, 1996, 1996), alternative amines (Cohen and Chao, 1968; Antonucci and Venz, 1987), and alternative curing devices (Tarle et al., 1995; Puppala et al. 1996). Peutzfeldt and Asmussen (1996, 1996), seeking to increase crosslinking via additional components in the resin system, reported that diacetyl (2,3-butanedione) and propanal improved several properties in peroxide/amine initiated resins. Earlier, diacetyl had been reported to generate free radicals upon absorption of photons.
More recently, Makinen and Makinen (1982) and Inano et al. (1983) reported that several types of enzymes are photoxidized and inactivated in broad-spectrum visible light in the presence of a variety of alpha-dicarbonyl compounds such as 2,3-butanedione and 1-phenyl-1,2-propanedione. In visible light, 1-phenyl-1,2-propanedione inactivated enzymes more rapidly than the other diketones tested.
Thus, there is a need in the art for new photoinitiator systems for forming photo-cured dental resins which photopolymerize with higher efficiency and with less yellowing.